Modulating system



Jan. 27, 1931. c. A. CULVER 7 MODULATING SYSTEM Filed July 16, 1926 I NVEN TOR.

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Patented Jan. 27, 1931 UNITED STATES igaasit PATENT :oFFic CHARLES A.CULVER, or NORTHFIELD, MINNESOTA, ASSIGNOR, BY MESNE nssien mnnrrs, TOFEDERAL TELEGRAPH CQMPANY, A CORPORATION or CALIFORNIA MODULATING SYSTEMApplication filed July 16,

lation circuits for the output of a high frequency electricaloscillator, and more particularly to a system of modulation for use inconnection with radio broadcasting.

An object of this invention is to effect an improvement in the meanswhereby voice generated potentials may be made to control the highfrequency output of a radio transmitter, and more particularly toprovide a more flexible system whereby the potential on the oscillatorand modulatortubes may be independently controlled.

Another object is to reduce the amplitude of the non-signalling carrierwave.

Still another object of the invention is to reduce the non-signallingplate current in the transformer supplying plate potential to the powertubes to a minimum, thus avoiding distortion due to high flux density.

My invention will be more fully understood from the specificationhereinafter following by reference to the accompanying diagram whichshows the arrangement of a a radio transmitter employing the prlnclples5 the output of which is applied to the plate circuit of the oscillatorabove indicated through the transformer T This modulator organizationconsists of tubes VT and VT together with the usual accessory parts asshown. The condenser C is of low impedance to currents of audiofrequency and serves to insulate the grid of the power tube VT from theconstant plate potential but does, however, permit the passage of anycurrents of audio frequency which may obtain in the secondary of thetransformer T A common source of positive constant potential B suppliesboth the plates of the modulator tubes and the power tube. However, itwill be understood that the power tube and 1926. Serial No. 122,985.

the modulator tubes may have independent potential supplies if sodesired.

Voice generated potentials are impressed on the grid of the modulatortubes VT and VT by means of the microphone M actingin the usual mannerin the primary of the transformer T or through suitable audio frequencyamplifiers. The variations in the grid potential of the tubes VT and VT;thusefiected cause avariation in the plate current I through said tubesand hence through the primary P of the transformer T The ratio of theturns in the primary P and secondary S of the transformer T are sochosen that, because of the variations in the primary current I therewill be developed an alternating potential of audio frequency in thesecondary of said transformer of suitable valve topro-perly operate thepower tube VT Under'certain circumstances it may be found advisable toapply to the plate of the power tube VT a constant potential ofsuflicient magnitude to cause feeble oscillations to obtain. The exactvalue of this constant potential is in any given case controlled by theseries reistance lks shown in the drawings the high frequency power tubehas impressedupon it a relatively low direct current potential fromsource '13, this direct current potential being of a value just greatenough to maintain the tube in an oscillating condition. The workingpotential is supplied by the transformer T The input side of Tconstitutes the output circuitof a balanced amplifienbut the balancedfeature is not essential. ever, possesses certain advantages,particularly'in that it makes possible the. working of the iron at arelatively low non-signaling 7 flux density value. The chief point isthat the working anode potential is not supplied from a battery ordirect current generator but from a source of potential which gives analternating wave form corresponding to the modulating sound wave, andthat said potential is produced bymeans of aitube or tubes acting asVariable resistances in the primary of said transformers.

As a result of the operation of the various current and audio frequencyalternating current components of the plate currentof the power tube maybe adjusted independently of one anot ier. i

Another advantage of my invention is that,

: because of the balanced condition in the prihigh flux density will beabsent.

mary of the transformer T and the very low or zero direct current valuein the secondary,

the iron of the transformer T will be'worked at a relatively lowpermanent flux density and hence distortion of the wave form due to Inprevious attempts to design an arrangement whereby the plate of thepower tube might be supplied with an audio frequencypotential, there hasexisted a direct current component in the primary of the transformersupplying that potential. q

Still another advantage of this system of modulation is that, because ofthe very low constant plate current component in the power tube theamplitude of the carrier wave is maintained at an extremely low value,and in fact may, if desired, bereduced to zero. As a result of thiscondition the general interference caused by the non-signallingradiation is substantially reduced, and in addition the over-allefliciency of the transmitting organization is increased. I

Having described my invention in one of its preferred embodiments, Idesire that it be understood that modifications may be made and that nolimitations upon the invention are intended other than are imposed bythe scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of therUnitedStates is as follows: V

1. A modulation system comprisinga plurality of electron tubes eachhaving grid,;iilament andplate electrodes, input and output circuitsinterconnecting said electrodes with the circuits of one of saidelectron tubes mutually coupled and connected with a source of potentialfor the generation of high frequency oscillations, a transformer havingprimary and secondary windings with the secondary winding thereofconnected in circuit with; said first mentioned electron tube, one ofthe terminals of said secondary winding being connected directly to theplate electrode of said first mentioned electron tube, a microphonecircuit, a pair of said electron tubes being connected in balancedrelationship and disposed between said microphone circuit and theprimary winding of said transformer, whereby variable audio frequencycurrents may be impressed upon said first mentioned electron tube andsuperimposed upon the potential supplied by said source for controllingthe amplitude of the high frequency currents generated in the circuitsthereof, and means for controlling said source of potentialindependently of said variable'audio frequency currents.

2. A modulation system comprising a plurality of'electron tubes eachhaving grid, filament and plate electrodes, input and output circuitsinterconnecting said electrodes with the circuits of one of saidelectrontubes mutually coupled for the generation of high frequencyoscillations, a transformer having pri- 'mary and secondary windings,with the sec,

said source of potential being adjustably controlled independent'ofcurrent in said primary winding, said source connected wlth said mid-tapin said primary winding and with the output circuit of said first men-vtioned electron tube and operating to sustain oscillations at apredetermined amplitude whereby variable alternating current may beimpressed upon said first mentionedelectron tube and superimposed uponthe oscillations sustained therein for controlling the amplitude of saidoscillations in accordance with the operation of saidmicrophone'circuit.

3. Asystem of modulation comprising a plurality of electron tubes eachhaving'grid, filament andplate electrodes, input and-output circuitsinterconnecting said electrodes, with the circuits of one of said tubesmutual 1y coupled for the generation of high frequency oscillations, atransformer having pri mary and secondary windings, with the secondarywinding thereof connected in circuit with the first mentioned electrontube, a mi lie crophone circuit, a pair of electron tubes concontrollingthe value of the potential impressed upon said plate circuitindependently of controlling the intensity of the Variations in audiofrequency current supplied to said first mentioned electron tube byvirtueof the operation of said microphone circuit.

In testimony whereof I alfix my signature.

CHARLES A. CULVER.

